DC/DC converter circuits are used in many electronic devices. Their task is to generate one or more DC voltages from an input voltage furnished, for example, by a battery, to apply it to a load connected to the output of the DC/DC converter circuit and to supply the load with the necessary load current. To generate the various voltages and polarities required of the output voltages, use is made of various kinds of DC/DC converter circuits capable of converting the input voltage into a higher, lower or inverted output voltage. The salient design criteria of DC/DC converter circuits are high effectiveness over a wide range of the load current and a straight-forward configuration.
One basic DC/DC converter circuit of the UP converter type, i.e. for converting the input voltage into a higher output voltage is described, for example, in the German Semiconductor Circuit Textbook by U. Tietze and Ch. Schenk, published by Springer-Verlag, 12th edition, 2001 on pages 948–949. This DC/DC converter circuit includes an inductance whose one terminal is connected to the input of the DC/DC converter circuit and whose other terminal is connectable via a first controllable switch to GND and connected to the anode of a diode whose cathode is connected to the output of the DC/DC converter circuit.
Disclosed in German laid-open patent DE 19940419 A1 in FIG. 2 thereof is a circuit in which the diode is replaced by a second controllable switch to eliminate the energy losses caused by the diode.
To control the two switches, the DC/DC converter circuit requires a controller which is connected to the two switches. The controller generates control signals of the switches and serves in addition to regulate the output voltage. One such controller is described, for example, in the aforementioned Textbook by U. Tietze and Ch. Schenk on page 946.
Regulating the output voltage in this case is done via the period of the switching signals for the two switches. These are thereby controlled by means of a pulse width modulator circuit so that the DC/DC converter circuit can operate in two permanently alternating operating time phases.
During the first operating time phase, the first switch is closed and the second switch is open. This permits energy to flow from a voltage source arranged at the input to the inductance. When then in the second operating time phase the first switch is opened and the second switch is closed, energy flows from the inductance to a load applied to the output of the DC/DC converter circuit. During the two time phases, a continually rising and falling flow of current materializes through the inductance.
With the reduction in the load current, the resulting total current flow through the inductance must continually diminish so that during the second operating time phase the current flowing through the inductance can fall to zero or even below zero. This mode in which the current flow is interrupted is associated, however, with a relatively poor efficiency, due to a power loss materializing from the recharging of the switch gate capacitances in the permanent switching action. This is why the DC/DC converter circuit can also operate in a quiescent or “skip” mode in which there is no flow of energy to and from the inductance by both switches being open. Once the output voltage drops below a predefined value, the skip mode needs to be deactivated and the permanently alternating storage of energy in the inductance and output of the energy stored in the inductance recommences.
Deactivating the skip mode is usually detected by a comparator applied to the output of the DC/DC converter circuit which monitors the potential of the output voltage and compares it to a reference voltage. The requirement for activating the skip mode is thus a zero current status of the inductance, it being the direction of the flow of current in the inductance during the time phase t4 that dictates the skip mode being activated. Measuring the flow of current through the inductance in DC/DC converter circuits is done in prior art by an operational amplifier as evident from FIG. 2, for example, of DE 19940419 A1. The operational amplifier is connected by its inverting input to the one terminal of the second switch and by its non-inverting input to the other terminal of the second switch. The ohmic resistance of the second switch in the ON condition results in a drop in voltage as a function of the load current as detected by the operational amplifier.
However, analyzing these analog signals is problematic. Since in most embodiments of the DC/DC converter circuit, the ohmic resistance of the second switch in the ON condition is very low, a corresponding small drop in voltage needs to be detected. When, for example, the switch is configured as a MOSFET the resistance often amounts to but a few tenths of an ohm, thus requiring the operational amplifier to operate with a high gain. In addition, the offset voltage of the operational amplifier makes it difficult to precisely define the switching threshold. Possible deviations in the tolerance of the components involved in the operational amplifier likewise need to be taken into account. Safely engineering the switching action is thus only possible with complicated circuitry and often necessitating tailored calibration of the individual DC/DC converter circuits.